Abdominal radiation therapy is often dose-limited by the risk of intestinal toxicity (radiation enteropathy). Radiation enteropathy is characterized by epithelial barrier breakdown, mucosal inflammation, and progressive fibrosis. Mast cell hyperplasia and overexpression of fibrogenic cytokines, most notably TGF-beta1, are prominent features at the cellular and molecular levels. The PI recently demonstrated that mast cells and TGF-beta1 are both mechanistically involved in intestinal radiation fibrosis, but their fibrogenic effects appear to be mutually dependent; that protease-activated receptor 2 (PAR-2), which is activated by mast cell tryptase and mediates mitogenic and fibrogenic responses, is upregulated in intestinal radiation fibrosis; and that maintaining normal neuroimmune communication between mast cells and enteric sensory nerves appears to be critical for an appropriate radiation response. The proposed research will use validated, genetically modified animal models, along with quantitative molecular methods, to systematically dissect the mechanisms by which mast cells may modulate the intestinal radiation response through interactions with TGF- beta1, PAR-2, and enteric nerves. The project will 1) examine the putative role of mast cells in TGF-beta1 activation and stabilization; 2) assess, in vivo, the role of heparin as an important mast cell mediator in radiation enteropathy; 3) investigate the role of PAR-2 in the mechanism of intestinal radiation fibrosis; 4) examine the role of mast cells in the mechanisms by which sensory nerve ablation augments radiation enteropathy; and 5) assess whether a substance P receptor antagonist modulates radiation enteropathy and whether this effect is mast cell-dependent. These experiments will provide substantial new insight into the basic pathogenesis of the intestinal radiation response. A comprehensive understanding of these underlying mechanisms is critical for identifying novel targets for intervention. This project may facilitate development of specific strategies to minimize intestinal radiation toxicity, thereby making radiation therapy safer and more effective.